Fire crews are working through the night to try to gain the upper hand on major bushfires burning in New South Wales.

With fears the crisis could worsen over the next two days, 2,000 firefighters are being rotated through frontlines in the Blue Mountains in an attempt to prevent three major blazes from joining into a massive single front.

A state of emergency is in effect across the state as hundreds of reinforcements have arrived from interstate to bolster the effort.

Authorities are uncertain about weather conditions and what the next 48 hours may have in store for towns and communities under threat.

An emergency warning is in place for the State Mine fire near Lithgow, while watch and act alerts remain for blazes near Mount Victoria, Springwood and the Hall Road fire in the Southern Highlands.